1. Field of the Invention
The invention relates generally to the use of shells or housings for sources or sensors, particularly for subsurface measurements.
2. Background Art
Various well logging techniques are known in the field of hydrocarbon exploration and production. These techniques typically employ logging instruments or “sondes” equipped with sources adapted to emit energy through a borehole traversing the subsurface formation. The emitted energy interacts with the surrounding formation to produce signals that are detected and measured by one or more sensors on the instrument. By processing the detected signal data, a profile or “log” of the formation properties is obtained.
Logging techniques known in the art include “wireline” logging, logging-while-drilling (LWD), and logging-while-tripping (LWT). Wireline logging entails lowering the instrument into the borehole at the end of an electrical cable to obtain the subsurface measurements as the instrument is moved along the borehole. LWD entails disposing the instrument in a drilling assembly for use while a borehole is drilled through earth formations. LWT involves disposing sources or sensors within the drill string to obtain measurements while the string is withdrawn from the borehole.
Conventional electromagnetic (EM) logging instruments are implemented with antennas that are operable as sources and/or sensors. The antennas are generally coils of the cylindrical solenoid type comprised of one or more turns of insulated conductor wire wound around a support. In operation, the transmitter antenna is energized by an alternating current to emit EM energy through the borehole fluid (also referred to as “mud”) used in the drilling operation and into the formation. The signals detected at the receiver antenna reflect interaction with the mud and the formation. It is well known that measurements obtained by induction-type logging are affected by direct transmitter-to-receiver coupling. Thus with induction-type logging, the instrument is typically equipped with one or more “bucking” antennas disposed near the transmitter or receiver to eliminate or reduce these coupling effects.
The antennas are typically mounted with their axes along the longitudinal instrument axis. Thus, these instruments are implemented with antennas having longitudinal magnetic dipoles (LMD). When such an antenna is placed in a borehole and energized to transmit EM energy, currents flow around the antenna in the borehole and in the surrounding formation. There is no net current flow up or down the borehole. However, with instruments incorporating antennas having tilted or transverse coils, i.e., where the antenna's axis is not parallel to the support axis, a current flow exists under certain circumstances. These instruments are implemented with antennas having a transverse or tilted magnetic dipole (TMD). Some TMD antennas are configured with multiple coils. One TMD antenna design comprises a set of three coils, known as a triaxial antenna. Logging instruments equipped with TMDs are described in U.S. Pat. Nos. 4,319,191, 5,508,616, 5,757,191, 5,781,436, 6,044,325, 6,147,496.
A particularly troublesome property of the TMD is the extremely large borehole effect that occurs in high contrast situations, i.e., when the mud in the borehole is much more conductive than the formation. When a TMD is placed in the center of a borehole, there is no net current along the borehole axis. When it is eccentered in a direction parallel to the direction of the magnetic moment, the symmetry of the situation insures that there is still no net current along the borehole axis. However, when a TMD is eccentered in a direction perpendicular to the direction of the magnetic moment, axial currents are induced in the borehole. FIG. 1 illustrates the different TMD eccentricities within the borehole. In high contrast situations these currents can flow for a very long distance along the borehole. When these currents pass by TMD antennas, they can cause undesired corruptive signals that are many times larger than would appear in a homogeneous formation without a borehole. FIG. 2 illustrates this axial current flow encountered along the borehole with a non-conductive instrument. Further description of this “borehole effect” is found in U.S. Pat. Nos. 6,573,722, 6,556,015 and 6,541,979.
In wireline applications, the antennas are typically enclosed by a housing constructed of a tough plastic material composed of a laminated fiberglass material. In LWD applications, the antennas are generally mounted on a metallic support to withstand the hostile environment and conditions encountered during drilling. Conventional logging instruments are also being constructed of thermoplastic materials. The thermoplastic composite construction of these instruments provides a non-conductive structure for mounting the antennas. U.S. Pat. Nos. 6,084,052, 6,300,762, 5,988,300, 5,944,124 and GB 2337546 describe implementations of composite-based instruments and tubulars for oilfield operations.
Techniques to reduce or correct for induced axial currents due to TMD antennas are emerging in the field. U.S. Pat. Nos. 5,041,975 and 5,058,077 describe techniques for processing signal data from downhole measurements in an effort to correct for borehole effects or to compensate for the effect of eccentric rotation on the sensor while drilling. U.S. Pat. No. 4,651,101 describes a logging sonde configured to cancel electric fields on the sonde surface. Additional techniques for addressing the borehole effect are found in U.S. Pat. Nos. 6,573,722, 6,556,015, and 6,541,979. These techniques involve complicated signal processing and/or the implementation of additional components to the instruments in order to address the currents. Thus there remains a need for improved techniques to handle these undesired borehole currents.